Anti-theft systems for sensing the physical presence, or absence, of an apparatus typically rely on an electrical circuit having contacts touching an enclosure, usually connected to ground, which houses the apparatus. For example, a car radio may have a contact thereon which completes an electrical alarm circuit by touching a frame on an automotive dashboard into which the radio is inserted, as shown in U.S. Pat. No. 4,805,233 (Robitschko et al.). Lear-spring type contacts for completing an electrical alarm circuit are known which may be attached either to the radio or to the enclosure, such as disclosed in U.S. Pat. No. 4,679,026 (Knakowswi et al.).
Use of a lear-spring type contact in an anti-theft system to sense the presence of a lock assembly in a lock housing of an automotive ignition system was shown in U.S. Pat. No. 4,803,460 (Rhee et al.), assigned to the assignee of the present invention and incorporated herein by reference. As seen in FIG. 8, the anti-theft system of that patent employs a resilient contact 124 which is compressed against the internal surface of a housing 150 when a lock assembly 140 is inserted therein to provide a continuous ground for a resistor 122 in a circuit (not shown). Upon forced removal of the lock 140 from the housing 150, the contact 124 breaks contact with the housing 150 causing an interruption in the electrical path through the resistor 122 and triggering the circuit to activate a conventional alarm system, for example, an audio or visual alarm.
The contacts in prior apparatus presence sensing anti-theft systems generally serve both as a mechanical sensor, which senses the physical presence of the apparatus, and an electrical sensor, which provides electrical continuity or discontinuity to an alarm circuit. One disadvantage of these sensing assemblies is that the contact surfaces may be exposed to oil, grease, dirt, and other surface coating substances, thus inhibiting electrical contact therebetween. In addition, galvanic corrosion, perhaps induced by current flowing through the actuation mechanism, can degrade contact pressure between the contact surfaces when made of dissimilar metals. Either of these problems may trigger false alarms.
A particular problem with anti-theft systems for vehicle ignitions systems is that they typically are grounded to the steering column casing and require a screw and lug to complete the ground path. Such a design not only increases assembly and disassembly time, but also has other disadvantages. For example, the ground path through the steering column will vary between vehicle models due to differences in column design, column material properties, and method of attachment to the vehicle, thus requiring accomodation of the screw and lug. Additionally, the ground impedance provided by the screw and lug may change over time due to galvanic corrosion or due to the steering column not adequately connected to the chassis. Finally, the steering column with a screw and lug or other like system is subject to corrosion and loss of torque on the screw which will cause an open ground circuit and system failure.